The glutathione S-transferases appear to have three functions: They serve as catalysts in reactions in which glutathione is used to attack a vast number of compounds with a susceptible electrophilic atom; they serve as binding proteins for an even larger number of compounds that include bilirubin and heme; they serve as one of the reactants in the formation of covalent complexes with very highly reactive reagents including those with carcinogenic activity. All three functions are under active investigation.